1. Field of the Invention
The present invention relates to a one-end closed ceramic double tube comprising an inner cylinder and an outer cylinder connected with each other by a support portion to form an integral structure, and to a method of manufacturing the same.
2. Related Art Statement
One-end closed ceramic double tubes comprising an inner cylinder and an outer cylinder have heretofore been put to diversified uses, for example, various filters, heat exchangers, condensers, support tubes for solid oxide fuel cells, and the like.
FIG. 12 shows an example of conventional one-end closed ceramic double tubes having been used as a support tube for solid oxide fuel cells. In FIG. 12, for example, a solid oxide fuel cell comprises an air chamber 26, an exhaust gas chamber 27, a generating chamber 28 and a fuel chamber 29, arranged in series. An upper plate 23 defines the air chamber 26 and the exhaust gas chamber 27; a middle plate 25 defines the exhaust gas chamber 27 and the generating chamber 28; and a bottom plate 24 defines the generating chamber 28 and the fuel chamber 29. The middle plate 25 and the bottom plate 24 are provided with gas outlets 31 and fuel inlets 30, respectively. The support tube for solid oxide fuel cells has a one-end closed double tube structure which comprises a one-end closed, zirconia outer cylinder 22 and a separate inner cylinder 21 coaxially inserted therein. The outer cylinder 22 is provided on its outer peripheral surface with an electrode, a solid electrolyte layer, etc. not shown in this figure. This outer cylinder 22 is supported at its closed bottom end by the bottom plate 24 and self-supported by being connected with a juxtaposed, adjacent double tubes' outer cylinders, not shown in this figure, by the surface electrode or interconnects. Further, the inner cylinder 21 is supported on the upper plate 23 by engagement therewith of a flange 21a provided on the upper portion of the inner cylinder.
An oxidizing gas, such as air or the like, is supplied from the air chamber 26 into the inner cylinder 21. Turning back at the closed end of the outer cylinder 22, the oxidizing gas returns through a passageway between the outer periphery of the inner cylinder 21 and the inner periphery of the outer cylinder 22, and flows out into the exhaust gas chamber 27. On the other hand, a fuel gas, such as H.sub.2, CH.sub.4 or the like, fed through the fuel inlets 30 of the bottom plate 24, flows along the outer periphery of the outer cylinder 22, forming oxygen ions which migrate through the zirconia solid electrolyte. As a result, an electric current flows between an inner air electrode or cathode provided on the outer periphery of the outer cylinder 22 and an outer fuel electrode or anode provided on almost the entire periphery of the solid electrolyte, through an interconnect between the inner electrode of one outer cylinder and the outer electrode of an adjacent outer cylinder. Thus, the solid oxide fuel cell works as a battery. Since the fuel cells are used at a high temperature of about 1,000.degree. C., one-end closed ceramic double tubes such as shown in FIG. 12, which can be jointlessly constructed, can be said to be preferable embodiments.
However, the one-end closed ceramic double tubes having the above-described construction have a problem in that it is difficult to achieve the positioning of the inner cylinder 21 within the outer cylinder 22, since the inner cylinder 21 and the outer cylinder 22 are separate bodies, and maintenance of the inner cylinder 21 is only attained by engagement of the flange 21a with the upper plate 23.
Further, as a result of the difficult positioning, the inner cylinder 21 assumes various positions within the outer cylinder 22, so that the flow rate of the oxidizing gas, such as air, varies accordingly, when the gas supplied through the inner cylinder 21 turns back at the closed end and goes up through the passageway between the outer periphery of the inner cylinder 21 and the inner periphery of the outer cylinder 22, causing problems such as uneven performances of each cell.
Moreover, since the inner cylinder 21 and the outer cylinder 22 are separate bodies, when the one-end closed ceramic double tubes having the above-described construction are used, for example, as a support tube for solid oxide fuel cells, it is difficult to obtain a sufficient mechanical strength to endure vibration or the like at the time of mounting and during operation.
Meanwhile, in order to solve the above problems, it may be conceivable to form the inner cylinder and outer cylinder into an integral body interposing a support portion therebetween. In this case, however, an extrusion molding into an integral body is required, which presents a problem such that a screw continuous extruder cannot make the outer and inner cylinders different in density. This is particularly a significant problem when the double tubes are used as a support tube for solid oxide fuel cells wherein the inner cylinder requires to be dense to gas-tight and the outer cylinder requires to be porous to gas permeation.
Further, in U.S. Pat. No. 4,751,152, there is proposed a porous, self-supporting, elongated electrode having a closed end, an inner wall defining a gas feed conduit, and at least one other gas exit conduit within the structure, where the conduits communicate with each other near the closed end of the structure. In this electrode, however, the support is comprised of an expensive electrode material in place of conventionally employed inexpensive materials, so that the cost of the whole electrode becomes so high that an economical disadvantage is unavoidable.
Furthermore, when the electrolyte and interconnect are provided on a part of the outer cylinder by means of, for example, chemical vapor phase deposition or the like, a selected portion on which the interconnect is provided has to be masked during deposition of the electrolyte to provide a discontinuity and then the electrolyte deposited portion has to be masked during deposition of an interconnect material on the discontinuity. This is very troublesome work.